Matching Items (27)
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- All Subjects: Materials Science
- All Subjects: Inorganic Separator
- Creators: Chan, Candace
Description
Dealloying, the selective dissolution of an elemental component from an alloy, is an important corrosion mechanism and a technological significant means to fabricate nanoporous structures for a variety of applications. In noble metal alloys, dealloying proceeds above a composition dependent critical potential, and bi-continuous structure evolves "simultaneously" as a result of the interplay between percolation dissolution and surface diffusion. In contrast, dealloying in alloys that show considerable solid-state mass transport at ambient temperature is largely unexplored despite its relevance to nanoparticle catalysts and Li-ion anodes. In my dissertation, I discuss the behaviors of two alloy systems in order to elucidate the role of bulk lattice diffusion in dealloying. First, Mg-Cd alloys are chosen to show that when the dealloying is controlled by bulk diffusion, a new type of porosity - negative void dendrites will form, and the process mirrors electrodeposition. Then, Li-Sn alloys are studied with respect to the composition, particle size and dealloying rate effects on the morphology evolution. Under the right condition, dealloying of Li-Sn supported by percolation dissolution results in the same bi-continuous structure as nanoporous noble metals; whereas lattice diffusion through the otherwise "passivated" surface allows for dealloying with no porosity evolution. The interactions between bulk diffusion, surface diffusion and dissolution are revealed by chronopotentiometry and linear sweep voltammetry technics. The better understanding of dealloying from these experiments enables me to construct a brief review summarizing the electrochemistry and morphology aspects of dealloying as well as offering interpretations to new observations such as critical size effect and encased voids in nanoporous gold. At the end of the dissertation, I will describe a preliminary attempt to generalize the morphology evolution "rules of dealloying" to all solid-to-solid interfacial controlled phase transition process, demonstrating that bi-continuous morphologies can evolve regardless of the nature of parent phase.
ContributorsChen, Qing (Author) / Sieradzki, Karl (Thesis advisor) / Friesen, Cody (Committee member) / Buttry, Daniel (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Solid-state lithium-ion batteries are a major area of research due to their increased safety characteristics over conventional liquid electrolyte batteries. Lithium lanthanum zirconate (LLZO) is a promising garnet-type ceramic for use as a solid-state electrolyte due to its high ionic conductivity. The material exists in two dierent phases, one that is cubic in structure and one that is tetragonal. One potential synthesis method that results in LLZO in the more useful, cubic phase, is electrospinning, where a mat of nanowires is spun and then calcined into LLZO. A phase containing lanthanum zirconate (LZO) and amorphous lithium occursas an intermediate during the calcination process. LZO has been shown to be a sintering aid for LLZO, allowing for lower sintering temperatures. Here it is shown the eects of internal LZO on the sintered pellets. This is done by varying the 700C calcination time to transform diering amounts of LZO and LLZO in electrospun nanowires, and then using the same sintering parameters for each sample. X-ray diraction was used to get structural and compositional analysis of both the calcined powders and sintered pellets. Pellets formed from wires calcined at 1 hour or longer contained only LLZO even if the calcined powder had only undergone the rst phase transformation. The relative density of the pellet with no initial LLZO of 61.0% was higher than that of the pellet with no LZO, which had a relative density of 57.7%. This allows for the same, or slightly higher, quality material to be synthesized with a shorter amount of processing time.
ContributorsLondon, Nathan Harry (Author) / Chan, Candace (Thesis director) / Tongay, Sefaattin (Committee member) / Department of Physics (Contributor) / Materials Science and Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2017-05
Description
Topological insulators with conducting surface states yet insulating bulk states have generated a lot of interest amongst the physics community due to their varied characteristics and possible applications. Doped topological insulators have presented newer physical states of matter where topological order co&ndashexists; with other physical properties (like magnetic order). The electronic states of these materials are very intriguing and pose problems and the possible solutions to understanding their unique behaviors. In this work, we use Electron Energy Loss Spectroscopy (EELS) – an analytical TEM tool to study both core&ndashlevel; and valence&ndashlevel; excitations in Bi2Se3 and Cu(doped)Bi2Se3 topological insulators. We use this technique to retrieve information on the valence, bonding nature, co-ordination and lattice site occupancy of the undoped and the doped systems. Using the reference materials Cu(I)Se and Cu(II)Se we try to compare and understand the nature of doping that copper assumes in the lattice. And lastly we utilize the state of the art monochromated Nion UltraSTEM 100 to study electronic/vibrational excitations at a record energy resolution from sub-nm regions in the sample.
ContributorsSubramanian, Ganesh (Author) / Spence, John (Thesis advisor) / Jiang, Nan (Committee member) / Chen, Tingyong (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Recent advancements in the field of light wavefront engineering rely on complex 3D metasurfaces composed of sub-wavelength structures which, for the near infrared range, are challenging to manufacture using contemporary scalable micro- and nanomachining solutions. To address this demand, a novel parallel micromachining method, called metal-assisted electrochemical nanoimprinting (Mac-Imprint) was developed. Mac-Imprint relies on the catalysis of silicon wet etching by a gold-coated stamp enabled by mass-transport of the reactants to achieve high pattern transfer fidelity. This was realized by (i) using nanoporous catalysts to promote etching solution diffusion and (ii) semiconductor substrate pre-patterning with millimeter-scale pillars to provide etching solution storage. However, both of these approaches obstruct scaling of the process in terms of (i) surface roughness and resolution, and (ii) areal footprint of the fabricated structures. To address the first limitation, this dissertation explores fundamental mechanisms underlying the resolution limit of Mac-Imprint and correlates it to the Debye length (~0.9 nm). By synthesizing nanoporous catalytic stamps with pore size less than 10 nm, the sidewall roughness of Mac-Imprinted patterns is reduced to levels comparable to plasma-based micromachining. This improvement allows for the implementation of Mac-Imprint to fabricate Si rib waveguides with limited levels of light scattering on its sidewall. To address the second limitation, this dissertation focuses on the management of the etching solution storage by developing engineered stamps composed of highly porous polymers coated in gold. In a plate-to-plate configuration, such stamps allow for the uniform patterning of chip-scale Si substrates with hierarchical 3D antireflective and antifouling patterns. The development of a Mac-Imprint system capable of conformal patterning onto non-flat substrates becomes possible due to the flexible and stretchable nature of gold-coated porous polymer stamps. Understanding of their mechanical behavior during conformal contact allows for the first implementation of Mac-Imprint to directly micromachine 3D hierarchical patterns onto plano-convex Si lenses, paving the way towards scalable fabrication of multifunctional 3D metasurfaces for applications in advanced optics.
ContributorsSharstniou, Aliaksandr (Author) / Azeredo, Bruno (Thesis advisor) / Chan, Candace (Committee member) / Rykaczewski, Konrad (Committee member) / Petuskey, William (Committee member) / Chen, Xiangfan (Committee member) / Arizona State University (Publisher)
Created2022
Description
This work correlates microscopic material changes to short- and long-term performance in modern, Cu-doped, CdTe-based solar cells. Past research on short- and long-term performance emphasized the device-scale impact of Cu, but neglected the microscopic impact of the other chemical species in the system (e.g., Se, Cl, Cu), their distributions, their local atomic environments, or their interactions/reactions. Additionally, technological limitations precluded nanoscale measurements of the Cu distributions in the cell, and microscale measurements of the material properties (i.e. composition, microstructure, charge transport) as the cell operates. This research aims to answer (1) what is the spatial distribution of Cu in the cell, (2) how does its distribution and local environment correlate with cell performance, and (3) how do local material properties change as the cell operates? This work employs a multi-scale, multi-modal, correlative-measurement approach to elucidate microscopic mechanisms. Several analytical techniques are used – including and especially correlative synchrotron X-ray microscopy – and a unique state-of-the-art instrument was developed to access the dynamics of microscopic mechanisms as they proceed. The work shows Cu segregates around CdTe grain boundaries, and Cu-related acceptor penetration into the CdTe layer is crucial for well-performing cells. After long-term operation, the work presents strong evidence of Se migration into the CdTe layer. This redistribution correlates with microstructural changes in the CdTe layer and limited charge transport around the metal-CdTe interface. Finally, the work correlates changes in microstructure, Cu atomic environment, and charge collection as a cell operates. The results suggest that, as the cell ages, a change to Cu local environment limits charge transport through the metal-CdTe interface, and this change could be influenced by Se migration into the CdTe layer of the cell.
ContributorsWalker, Trumann (Author) / Bertoni, Mariana I (Thesis advisor) / Holman, Zachary (Committee member) / Chan, Candace (Committee member) / Colegrove, Eric (Committee member) / Arizona State University (Publisher)
Created2022
Description
The current Li-ion batteries with organic liquid electrolytes are limited by their safety and energy density. Therefore, ceramic electrolytes are proposed in developing next-generation, energy-dense Li-metal batteries by replacing organic liquid electrolytes to improve safety and performance. Among numerous ceramic Li-ion conductors, garnet-based solid electrolyte c-Li7La3Zr2O12 (c-LLZO) is considered one of the most promising candidates to enable Li metal batteries due to its high ionic conductivity, chemical stability, and wide electrochemical stability window against Li metal. However, synthesis and processing of c-LLZO through conventional solid-sate reaction methods requires long periods of calcination (> 6 h) at high reaction temperatures (> 1000 °C). The need for high reaction temperature results to attain cubic-LLZO phase results in large aggerated LLZO particles and causes Li-loss from the garnet structure, making them unfavorable to process further as bulk pellets or thin films. To overcome processing challenges with solid-state reaction method, two novel facile synthesis approaches molten salt (flux growth method), and solution combustion are employed to produce submicron-sized LLZO powders at low reaction temperatures (< 1000 °C) in a short time. In the first case, molten salt synthesis method with LiCl-KCl eutectic mixture is employed to produce sub-micron sized Ta-doped LLZO (LLZTO) powders at low temperatures (900 °C, 4 h). In addition, a detailed investigation on effect of sintering medium and sintering additives on the structural, microstructural, chemical, and Li-ion transport behavior of the LLZTO pellets are investigated. Sintered LLZTO pellets prepared using molten salt synthesis route exhibited high Li-ion conductivity up to 0.6 mS cm-1 and high relative density (> 95 %) using Pt-crucible. In the second case, a facile solution-combustion technique using an amide-based fuel source CH6N4O is utilized to produce submicron-sized Al-doped LLZO (Al-LLZO) powders at low reaction temperatures 600-800 °C in a short duration of 4 h. In addition, effect of fuel to oxidizer ratio on phase purity, particle growth size, and formation mechanism of conductive Al-LLZO are reported and discussed. The Al-LLZO pellets sintered at 1100 °C/ 6 h exhibited high Li-ion conductivity up to 0.45 mS cm-1 with relative densities (> 90 %).
ContributorsBadami, Pavan Pramod (Author) / Kannan, Arunachalandar Mada (Thesis advisor) / Chan, Candace (Thesis advisor) / Song, Kenan (Committee member) / Arizona State University (Publisher)
Created2021
Description
Hydrogen is considered one of the most potential fuels due to its highest gravimetric energy density with no pollutant emission during the energy cycle. Among several techniques for hydrogen generation, the promising photoelectrochemical water oxidation is considered a long-term solar pathway by splitting water. The system contains a photoanode and a cathode immersed in an aqueous electrolyte where charge separation takes place in the bulk of the semiconducting material on light absorption, leading to water oxidation/reduction at the surface of the photoelectrodes/cathode. It is imperative to develop materials that demonstrate high light absorption in the wide spectrum along with photoelectrochemical stability. N-type Monoclinic scheelite bismuth vanadate (BiVO4) is selected due to its incredible light absorption capabilities, direct bandgap (Eg ∼ 2.4-2.5 eV) and relatively better photoelectrochemical stability. However, BiVO4 encounters huge electron-hole recombination due to smaller diffusion lengths and positive conduction bands that cause slow charge dynamics and sluggish water oxidation kinetics. In order to improve the illustrated drawbacks, four strategies were discussed. Chapter 1 describe the fundamental understanding of photoelectrochemical cell and BiVO4. Chapter 2 illustrates details of the experimental procedure and state-of-the-art material characterization. Chapter 3 provide the impact of alkali metal placement in the crystal structure of BiVO4 systematically that exhibited ~20 times more performance than intrinsic BiVO4, almost complete bulk charge separation and enhancement in the diffusion length. Detailed characterization determined that the alkali metal getting placed in the interstitial void of BiVO4 lattice and multiple interbands formation enhanced the charge dynamics. Chapter 4 contains stoichiometric doping of Y3+ or Er3+ or Yb3+ at the Bi3+ site, leading to an extended absorption region, whereas non-stoichiometric W6+ doping at the V5+ site minimizes defects and increased charge carriers. To further enhance the performance, type-II heterojunction with WO3 along p-n junction with Fe:NiO enhance light absorption and charge dynamics close to the theoretical performance. Chapter 5 provides a comprehensive study of a uniquely developed sulfur modified Bi2O3 interface layer to facilitate charge dynamics and carrier lifetime improvement by effectively passivating the WO3/BiVO4 heterojunction interface. Finally, chapter 6 summarized the major findings, conclusion and outlook in developing BiVO4 as an efficient photoanode material.
ContributorsPrasad, Umesh (Author) / Kannan, Arunachala Mada (Thesis advisor) / Azeredo, Bruno (Committee member) / Chan, Candace (Committee member) / Segura, Sergio Garcia (Committee member) / Arizona State University (Publisher)
Created2021
Description
Lithium nickel manganese cobalt oxides (NMCs) are layered oxide cathode materials which are becoming increasingly popular as the demand for lithium-ion batteries increases. Lithium-ion batteries are used to power modern vehicles and for other battery applications. To better understand the structure and energetics of NMCs, various molar ratios of these compounds were synthesized via a sol-gel method and characterized with powder X-ray diffraction profile fitting. Lattice constants for the nickel, manganese, and cobalt solid solutions were determined. High temperature oxide melt solution calorimetry was used to determine the enthalpies of formation and mixing. All but Li2MnO3 had the same space group as LiCoO2 (R-3m). The lattice constants approximately followed a linear fit with cobalt mole fraction (R2average= 0.973) for the cobalt series. As the molar ratio of cobalt increased the lattice constants decreased. The nickel series was less linear (R2average=0.733) and had an opposite lattice constant trend to cobalt. The manganese series possessed a roughly linear trend when excluding the outlier Li2MnO3 (R2average=0.282). The formation enthalpy of the cobalt series becomes more negative as more cobalt is added. A second order polynomial fit could be used to model the enthalpies of mixing for the series. NMC2.5,2.5,5 exhibited the most stable energetics. A third order polynomial fit could be used to model the enthalpy of mixing for the nickel and manganese series with NMC811 and NMC181 exhibiting the most stable energetics.
ContributorsKanitz, William James (Author) / Navrotsky, Alexandra (Thesis advisor) / Chan, Candace (Committee member) / Xu, Hongwu (Committee member) / Arizona State University (Publisher)
Created2023
Description
The metallization and interconnection of Si photovoltaic (PV) devices are among some of the most critically important aspects to ensure the PV cells and modules are cost-effective, highly-efficient, and robust through environmental stresses. The aim of this work is to contribute to the development of these innovations to move them closer to commercialization.Shingled PV modules and laser-welded foil-interconnected modules present an alternative to traditional soldered ribbons that can improve module power densities in a cost-effective manner. These two interconnection methods present new technical challenges for the PV industry. This work presents x-ray imaging methods to aid in the process-optimization of the application and curing of the adhesive material used in shingled modules. Further, detailed characterization of laser welds, their adhesion, and their effect on module performances is conducted. A strong correlation is found between the laser-weld adhesion and the modules’ durability through thermocycling. A minimum laser weld adhesion of 0.8 mJ is recommended to ensure a robust interconnection is formed.
Detailed characterization and modelling are demonstrated on a 21% efficient double-sided tunnel-oxide passivating contact (DS-TOPCon) cell. This technology uses a novel approach that uses the front-metal grid to etch-away the parasitically-absorbing poly-Si material everywhere except for underneath the grid fingers. The modelling yielded a match to the experimental device within 0.06% absolute of its efficiency. This DS-TOPCon device could be improved to a 23.45%-efficient device by improving the optical performance, n-type contact resistivity, and grid finger aspect ratio.
Finally, a modelling approach is explored for simulating Si thermophotovoltaic (TPV) devices. Experimentally fabricated diffused-junction devices are used to validate the optical and electrical aspects of the model. A peak TPV efficiency of 6.8% is predicted for the fabricated devices, but a pathway to 32.5% is explained by reducing the parasitic absorption of the contacts and reducing the wafer thickness. Additionally, the DS-TOPCon technology shows the potential for a 33.7% efficient TPV device.
ContributorsHartweg, Barry (Author) / Holman, Zachary (Thesis advisor) / Chan, Candace (Committee member) / Bertoni, Mariana (Committee member) / Yu, Zhengshan (Committee member) / Arizona State University (Publisher)
Created2023
Description
Electrochemical technologies emerge as a feasible solution to monitor and treat pollutants. Although electrochemical technologies have garnered widespread attention, their commercial applications are still constrained by the use of expensive electrocatalysts, and the bulky and rigid plate design of electrodes that restricts electrochemical reactor design to systems with poor electrode surface/ volume treated ratios. By making electrodes flexible, more compact designs that maximize electrode surface per volume treated might become a reality. This dissertation encompasses the successful fabrication of flexible nanocomposite electrodes for electrocatalysis and electroanalysis applications.First, nano boron-doped diamond electrodes (BDD) were prepared as an inexpensive alternative to commercial boron-doped diamond electrodes. Comparative detailed surface and electrochemical characterization was conducted. Empirical study showed that replacing commercial BDD electrodes with nano-BDD electrodes can result in a cost reduction of roughly 1000x while maintaining the same electrochemical performance.
Next, self-standing electrodes were fabricated through the electropolymerization of conducing polymer, polypyrrole. Surface characterizations, such as SEM, FTIR and XPS proved the successful fabrication of these self-standing electrodes. High mechanical stability and bending flexibility demonstrated the ability to use these electrodes in different designs, such as roll-to-roll membranes. Electrochemical nitrite reduction was employed to demonstrate the viability of using self-standing nanocomposite electrodes for electrocatalytic applications reducing hazardous nitrogen oxyanions (i.e., nitrite) towards innocuous species such as nitrogen gas. A high faradaic efficiency of 78% was achieved, with high selectivity of 91% towards nitrogen gas.
To further enhance the conductivity and charge transfer properties of self-standing polypyrrole electrodes, three different nanoparticles, including copper (Cu), gold (Au), and platinum (Pt), were incorporated in the polypyrrole matrix. Effect of nanoparticle wt% and interaction between metal nanoparticles and polypyrrole matrix was investigated for electroanalytical applications, specifically dopamine sensing. Flexible nanocomposite electrodes showed outstanding performance as electrochemical sensors with PPy-Cu 120s exhibiting a low limit of detection (LOD) of 1.19 µM and PPy-Au 120s exhibiting a high linear range of 5 µM - 300 µM.
This dissertation outlines a method of fabricating self-standing electrodes and provides a pathway of using self-standing electrodes based on polypyrrole and polypyrrole-metal nanocomposites for various applications in wastewater treatment and electroanalytical sensing.
ContributorsBansal, Rishabh (Author) / Garcia-Segura, Sergio (Thesis advisor) / Westerhoff, Paul (Committee member) / Perreault, Francois (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2023